As the decreased amount and the elevated price of gasoline have become a global issue, electric vehicles are the best solution for solving this problem. In the state of California of the United States of America, legislations are passed for forcing car dealers to sell a specific ratio of electric vehicles, where the other states will soon follow. France, Germany, Switzerland and Japan all have policies for rewarding the use or technical research of electric vehicles. Moreover, practical electric vehicles have been developed and gradually introduced in experimental trials.
Advanced countries such as the United States of America, Japan, and countries in Europe all pay high attention to the air pollution caused by urban transportation and the decrease in the amount of gasoline. Moreover, it is currently agreed to be a good time for developing the adoption of electric vehicles and putting more effort in the research and promotion of electric vehicles. In the state of California of the U.S., legislations are passed for forcing car dealers to sell a specific ratio of electric vehicles, where the other states will soon follow. France, Germany, Switzerland and Japan all have policies for rewarding the use or technical research of electric vehicles. Moreover, practical electric vehicles have been developed and gradually introduced in experimental trials. Here, lithium battery directing is the key to the successful development of electric vehicles. This is due to the fact that a lithium battery only weighs half of a nickel-hydrogen battery, but has an endurance two times longer than that of a nickel-hydrogen battery. Moreover, the lithium battery has high working voltage, high energy density, long life-span, and is environmental friendly. In addition, waste gas will not be exhausted when the lithium battery is applied in vehicles, such that not only is energy saved and carbon level reduced, but the amount of gasoline used is also reduced. The replacement of ordinary batteries with chargeable lithium batteries is a major trend for vehicle manufacturers in the future.
The global retrieval of lithium batteries in SONY notebook computers in 2006 brings out the thermal runaway safety problem of lithium batteries. The battery module used in a notebook computer is constituted by only 3 to 8 18650 unit cells. If an electric vehicle is powered by 18650 unit cells, 4000 to 6000 batteries would be required to provide sufficient power (dynamics) and capacitance (endurance). The increase in the number of batteries increases the chance of thermal runaway, and the thermal runaway in one of the batteries in the battery module may diffuse to the entire battery module. Once thermal runaway occurs in one of the batteries in the battery module, the battery cannot be controlled effectively and the thermal runaway diffuses gradually from the closest battery, such that thermal runaway occurs in the entire battery module, which is as dangerous as a small explosion.
Thus, the National Renewable Energy Laboratory (NREL) in the U.S.A. has recently performed a series of researches in the thermal runaway diffusion of the lithium battery module. The thermal runaway diffusion occurs when one of the batteries in the battery module releases heat abnormally due to short-circuit or uneven electricity (low volume or high internal resistance) in the charging/discharging process. Once the battery exceeds a threshold temperature for the thermal runaway reaction (usually about 150° C.), the material in the battery gradually goes through the thermal decomposition exothermic reaction. The so-called thermal decomposition exothermic reaction is a self-heating autocatalytic reaction, where the temperature of the battery is increased rapidly. When the thermal runaway occurs, the temperature of the battery can exceed 500° C. and the self-heating temperature increase is more than 20° C. per minute. Therefore, after exceeding the threshold thermal runaway temperature, the battery increases its temperature rapidly so as to result in thermal runaway. The thermal energy released from the thermal runaway of this battery then heats up the neighboring batteries if good insulation and heat dissipation structures were not designed. For example, as shown in the left diagram in page 30 of “Thermal Abuse Modeling of Li-ion Cells and Propagation in Module,” which is published in the 4th International Symposium of Large Lithium Ion Battery Technology and Applications (LLIBTA) held in May, 2008, the battery with thermal runaway causes the neighboring batteries to result in thermal runaway. As the thermal runaway inside the battery module reaches this stage, the thermal runaway cannot be controlled effectively, such that all of the other batteries in the entire battery module will all result in thermal runaway so as to generate combustion exothermic reactions. This process is usually followed by the release of large amounts of flammable electrolyte gas and battery material decomposition gas and more severely, possible explosions.
The thermal runaway safety problem of lithium batteries is mainly caused by overcharging and short-circuit. However, thermal runaway can also occur when the battery is punctured from an external impact. The thermal runaway of a battery is basically the reaction process of a battery internal material releasing heat under thermal degradation.
Currently, the control of thermal runaway diffusion has been disclosed in U.S. Pat. No. 6,942,944, U.S. Patent Application Publications No. US20060073377 and No. US20090004556. The above research teams are members involved in the research of battery thermal runaway in the NREL. In these disclosures, a phase change material is filled to spaces between the batteries, and the heat generated during the thermal runaway is absorbed using the heat absorbing property of the phase change process of the phase change material.
However, the above patents has a flaw; that is, the thermal conductivity of the phase change material is poor. The property can be used to insulate the heat transmission between the batteries during the thermal runaway. As the heat conductivity of the phase change material is poor, although the temperature increase of the battery module can be controlled during normal usage, the subsequent cooling rate requires longer time. For instance, in FIG. 9 of U.S. Pat. No. 6,942,944, it shows that after the battery module discharges, the battery module has to be placed still for almost 24 hours under a natural convention current for heat dissipation (without additional fans for heat dissipation) to return to the temperature before the discharging. The low thermal conductivity and heat absorbing property of the phase change material can be used to insulate the thermal runaway diffusion inside the battery module and reduce the temperature increase during charging/discharging. Nevertheless, the phase change material is unfavorable for its long cooling time, which is disadvantageous for the continuous charging/discharging of the battery module.